powerpc/spufs: Initialize ctx->stats.tstamp correctly

spuctx_switch_state() warns if ktime goes backwards, but it
sometimes compares an uninitialized value, which showed that
the data was unreliable when we actually saw the warning.

Initialize it to the current time in order to get correct data.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>